HP-41 Challenge: Double Integrals by INTEG Recursion

[Ángel Martin]

FOCAL code shown below. Includes 6 program steps to preserve and restore the Key assignments so they'll be unmodified at the end of the calculations.

Code:


1    LBL "2DITG"    
2    ASTO 05    function name
3    "KEYS"    
4    SAVEKA    sake keys
5    CLKEYS    clear ka space
6    STO 04    x2
7    RDN    
8    STO 03    x1
9    RDN    
10    STO 02    y2
11    RDN     
12    STO O1    y1
13    13    
14    B?         leftovers?
15    CLB        yes, delete
16    RCL 01    y1
17    RCL 02    y2
18    "*2D"    F(y)
19    INTEG    outer integral
20    "KEYS"    
21    GETKA    restore keys
22    CLA     
23    ARCL 05    function name
24    RTN        final result in X.
25    LBL "*2D"    
26    STO 00    y
27    14,013    
28    REIDBF    change buffer id#
29    RCL 03    x1
30    RCL 04    x2
31    CLA     
32    ARCL 05    function name
33    INTEG    inner integral
34    13,014    
35    REIDBF    change buffer id#
36    RDN            solution to X
37    END